When a vehicle travels on a cant road, which is a road inclined in the width direction, or in a crosswind, a behavior occurs in which the vehicle drifts on the cant road to the lower side of the inclined direction under the influence of gravity, and drifts in the crosswind on the leeward side by the influence of the wind. Such a behavior is called a unidirectional drift of a vehicle. In order to continue to drive the vehicle straight against the behavior of the unidirectional drift of the vehicle, a driver of the vehicle continuously needs to turn the steering wheel toward the upper side of the inclined direction or the windward side. That is, in a situation where a behavior of a unidirectional drift of a vehicle occurs, in order to maintain a straight traveling of the vehicle, a driver of the vehicle is required to continuously provide a steering effort in a direction opposite to the direction the vehicle drifts (the direction of the drift).
For example, in Japanese Patent Application Publication No. 2001-001923, a vehicle steering system is disclosed that obtains, when a disturbance effecting a vehicle behavior such as an inclination on a cant road or a crosswind occurs, i.e., when a vehicle travels on a cant road, in a crosswind, or the like, an effect value of disturbance which corresponds to a magnitude of an effect of the disturbance to the vehicle behavior, and then performs an assist torque control according to the steering in accordance with the influence value of disturbance, to cancel the effect of the disturbance to the vehicle behavior.